How do you calculate Array Modifier with Relative Offset?
As, i said, use constant offset instead relative, it's much easier:



"Relative" doesn't mean "length of object plus something", this means "length of object multiply by given number". Relative offset is a factor, that multiply with length of the object. In your case is 1.25 for top block and 1 for others. 

If want to calculate it, lets try:

Offset in m is 1.45 (1.25+0.2) relative unit is 1.25. So to get offset you need to divide 1.45 by 1.25

1.45/1.25=1.16


So, 1.16 is offset in relative units.

For bottom blocks with 1m length  relative units will be different (block has 1m length), so offset will be 1.45 / 1 = 1.45